This is, unfortunately, a known issue that we are hearing from some of our teachers and parents who use the Firefox browser. From some, the worksheets will default to Spanish usually without providing the option to switch over to English.
Although we are working to get this resolved, we don't yet have a resolution timeframe. Meanwhile, we can suggest that you use the most recent version of your browser since some have reported this seemed to help. If you aren't sure how to do that, you can check out this article right here: How to update my browser
If that does not work, we suggest using another browser such as Chrome since we know that the problem seems to be specific to Firefox.